This application is based on Japanese Patent Application No. 2001-249343 filed on Aug. 20, 2001 the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a discharge lamp ballast circuit for driving a high-tension discharge lamp, and is preferable for a vehicular headlamp using a discharge lamp.
2. Description of Related Art
Conventionally, a ballast circuit for a discharge lamp drives the discharge lamp to initiate an arc and to maintain the arc during a lighting period. In order to drive the discharge lamp efficiently, the ballast circuit controls power applied to the discharge lamp. For example, the ballast circuit has a DC/DC converter circuit to generate a high-tension voltage for driving the discharge lamp, and controls the DC/DC converter circuit to vary an output voltage. In order to reduce a switching loss of the DC/DC converter circuit, it is preferable to perform a soft switching.
JP-A-9-84343 discloses the ballast circuit capable of performing the soft switching. A capacitor connected in parallel with a switching device provides a snubber circuit for performing the soft switching when turning off the switching device.
According to the conventional ballast circuit the snubber circuit may effectively perform the soft switching when turning off the switching device. On the contrary, when turning on the switching device, the ballast circuit adjusts timing of turning on. However, waveform of the voltage and the current may vary in accordance with values of the inductance and capacitance of the circuit. For example, if the switching device is a MOS transistor that has a parasitic diode between the source and the drain, a resonance current may flow through the parasitic diode when the voltage on the MOS transistor reaches to a negative voltage due to a resonance and the resonance current may vary. The resonance current may cause a loss. Further, it is difficult to reduce the loss over before and after an initiation of the arc on the discharge lamp.
It is an object of the present invention to provide a discharge lamp ballast circuit which is capable of reducing loss.
It is an object of the present invention to provide a discharge lamp ballast circuit which is capable of reducing loss and supplying a sufficient voltage before the discharge lamp makes a breakdown.
It is still another object of the present invention to provide a discharge lamp ballast circuit that is capable of reducing a switching loss of the DC/DC converter circuit and has the DC/DC converter circuit easily up-converts before the breakdown of the discharge lamp.
According to a first aspect of the present invention, a discharge lamp ballast circuit has a capacitor that generates a resonance with a leakage reactor of a fly-back transformer of a DC/DC converter. The discharge lamp ballast circuit further has a control circuit for controlling a switching device of the DC/DC converter circuit in a PFM manner so that the switching device is driven in a non-zero voltage switching or a non-zero current switching during a lighting preparation period of the discharge lamp, and so that the switching device is driven in a zero voltage switching or a zero current switching after the lighting preparation period. The non-zero voltage switching or the non-zero current switching enables to increase the output voltage of the DC/DC converter circuit for satisfying a required voltage for initiating the discharge lamp. The resonance generated by the capacitor and the leakage reactor enables the zero voltage switching or the zero current switching to reduce switching loss. The capacitor or the leakage reactor provides a snubber circuit for the switching device of the DC/DC converter circuit. Therefore, it is possible to perform a soft switching by a snubber switching operation.
As a result, it is possible to supply sufficient voltage and to reduce switching loss.
The capacitor may be connected in parallel with the switching device or the primary coil of the fly-back transformer. In those cases, the capacitor performs a C-snubber switching operation. The capacitor may be connected in parallel with the switching device and the secondary coil of the fly-back transformer. In this case, the leakage reactor may perform an L-snubber switching operation. The switching device may be a semi-conductor switching device such as a MOS transistor.
The non-zero voltage switching may be performed in accordance with the voltage on the primary coil of the fly-back transformer. The switching device may be turned on after the voltage reaches to a peak value and before the voltage reaches to 0V or below 0V.
A switching frequency is variable due to the PFM control. The switching frequency takes higher value during the lighting preparation period than after the lighting preparation period. For example, the switching frequency is increased by the non-zero voltage switching or the non-zero current switching which are shortening an off or on period of the switching device. For example, the control circuit drives the switching device by a switching frequency when performing the zero voltage switching. The control circuit further drives the switching device by a higher switching frequency when performing the non-zero voltage switching in comparison with the switching frequency in the zero voltage switching. The higher switching frequency may be obtained by shortening an off period of the switching device. The off period is shortened by turning on the switching device earlier, for example, in a range after a voltage on the primary coil reaches to a peak value and before the voltage on the primary coil reaches to zero or below zero.